Much work has been done in the field of photopolymerization to increase the polymerization speed of these compositions. Many of the well-known photoinitiators or photoinitiator systems used, however, limit the applicability of the photopolymerizable compositions and elements because the initiators are activatible primarily in the ultraviolet region of the spectrum.
Chambers U.S. Pat. No. 3,479,185 discloses photopolymerizable compositions containing an ethylenically unsaturated monomer, a free radical producing agent such as a leuco triphenylamine dye and a hexaarylbiimidazole. These compositions are photoinitiatable in the ultraviolet region of the spectrum. Chambers, however, found that by adding energy-transfer dyes of the xanthene and acridine classes the sensitivity of the photopolymerizable compositions was extended into the visible spectral region with an increase in speed of polymerization.
Chang U.S. Pat. No. 3,549,367 discloses photopolymerizable compositions containing hexaarylbiimidazoles and p-aminophenyl ketones, e.g., Michler's ketone, which extend the spectral sensitivity of the compositions slightly into the visible region of the spectrum. Baum and Henry U.S. Pat. No. 3,652,275 further enhances the efficiency of the hexaarylbiimidazole systems in photopolymerizable compositions through the use of selected bis-(p-dialkylaminobenzylidene) ketone sensitizers.
While the aforementioned compositions have provided improved visible light sensitization of photopolymerizable compositions, further improvement in photo speed is desirable so that lower energy exposure sources can be used or the efficiency of imagewise polymerization exposure improved. Furthermore, improved sensitization enables the exposure source to be maintained at a greater distance from the photopolymerizable layer thereby allowing for greater collimation of radiation from the exposure source. It is also desirable to provide improved speed of sensitization in the visible region of the spectrum particularly for use in a photopolymerizable composition containing specific nitroaromatic compounds as photoinhibitors which are capable of producing positive polymeric images by means of a double exposure process, first imagewise in the ultraviolet region of the spectrum and then overall in the visible region of the spectrum. Nitroaromatic photoinhibitors and process for producing positive polymeric images are disclosed in Pazos, U.S. Ser. No. 758,699, filed Jan. 17, 1977.